Reduced bone cortical thickness in boys with autism or autism spectrum disorder.

Doctors took hand X-rays of boys with autism aged 4 to 8.

They measured the thickness of the outer bone layer, called cortical bone.

Some boys ate a casein-free diet. Others drank milk normally.

Autistic boys already had thinner bone walls than typical kids.

The gap got bigger every year.

Boys who avoided casein had almost twice the thinning.

Esteban-Figuerola et al. (2019) pooled many studies and found autistic kids eat less calcium and vitamin D. This gives a clear reason for the thin bones Matson et al. (2008) saw.

Elder et al. (2006) and Tonnsen et al. (2016) both ran blind trials and found no behavior gain from cutting casein. Their null results clash with the bone harm shown here, so the diet carries risk without proven benefit.

Öztürk et al. (2026) will review all GFCF papers. The bone warning from Matson et al. (2008) will sit in that review as a red-flag side effect.

Before you support a casein-free diet, weigh the bone cost. Check the child’s calcium and vitamin D intake. Ask parents to bring in a food log. If the diet continues, schedule pediatric bone checks and add fortified milk substitutes or supplements. Your clinical note can flag this risk for future providers.